redis: use UniveralClient, support sentinel mode

This commit switches the redis client to using a UniversalClient [1],
which can operate in standard, cluster, or failover (sentinel) mode.

It also implements basic support for having (multiple)
`redis+sentinel://` URLs as connection URL in the config (e.g. for
`redis-host`).

This is the Go equivalent of the proposed Python patch [2]. It supports
the same examples given there.

[1] https://pkg.go.dev/github.com/go-redis/redis/v8#UniversalClient
[2] https://lists.sr.ht/~sircmpwn/sr.ht-dev/patches/55521